1. Field of the Invention
The present invention relates to super-absorbents, to a process for their preparation and to their use in the hygiene sector.
2. Discussion of the Background
Super-absorbents, or highly water-absorbent and water-swellable polymers, are widely used in the sanitary and hygiene sector, in wallpaper pastes, as drying agents, as humectants in agriculture and as electrolyte thickeners in dry batteries. Known absorbents for these intended uses include synthetic polymers or starch graft polymers or cellulose graft polymers, the hydrophilicity of which is based on a high content of carboxyl groups. The fully synthetic polymers are usually polymers with a low degree of crosslinking such as partly crosslinked polyacrylic acid salts or partly crosslinked polymaleic acid derivatives. They have comparatively the best absorbent action, but are not degraded biologically.
Suitable graft polymers are derivatized polysaccharides in which water-soluble vinyl monomers are usually grafted therein. The graft polymers include the carboxymethyl cellulose, hydrolyzed starch/acrylonitrile graft polymers and acrylic acid/starch graft polymers. In comparison with the fully synthetic polymers, they display a significantly lower absorption capacity for water and aqueous liquids. However, the proportional biological degradation of these naturally occurring swelling agents is advantageous. Nevertheless, the preparation of such graft polymers is very expensive and the amount of biopolymer in the end product is severely limited by the high viscosity of the reaction medium, as is the case, for example, of a monomer solution of dissolved starch. Reference may be made in this respect, for example, to EP-A-0 168 390 and EP-A-0 188 489.
It is known from DE-A-35 05 920 that polymers or copolymers having a low degree of crosslinking, the hydrophilicity of which is based on a high content of cationic groups, are preferable to known anionic absorbents. In particular, they are less sensitive to salt solutions and display a significantly better absorption capacity. Suitable cationic absorbents are, inter alia, copolymers based on quaternary ammonium compounds and acrylamide, which can be polymerized by free radicals. These fully synthetic cationic polymers, like the fully synthetic anionic polymers, are not biologically degradable. A need therefore continues to exist for a biodegradable material of high absorbing capacity.